There have been many questions about the 540EZ relating to the
observation that it continues to make noises, even after it has
finished charging. Most users report their 540EZ makes some sort
of noise all the time. Mine seems to quietly "click" even after the
whine of the charging process has stopped. This all seems to be
"normal" behavior for the 540EZ. The 430EZ doesn't seem to do this.

A comment from Canon is as follows:

Q: Re. the 540EZ speedlite: Why does it make all sorts of clicking
noises after it's fully charged? Can it be "fixed"?

A: It's multiplexing between the capacitors for the main flashtube, the
auxiliary
(A-TTL preflash) flashtube, and the circuit that controls the LCD display and
motorized zoom head. Since this is a characteristic of the 540EZ's design,
there's no "fix" as such.

Q: Is there anything other than the "Owners Manual" which fully describes the
features of the 540EZ and it's interaction with current EOS bodies?

A: There's a new book on the 540EZ from Silver Pixel Press, a division of the
Saunders Group in Rochester, NY. It's co-authored by Peter Burian from
Shutterbug, and has a reasonable amount of detailed information on the product
as well as its usage. Call SPP at 716-328-7800 for more information.